Phase change memory device having an improved word line resistance, and methods of making same

ABSTRACT

A phase change memory device having an improved word line resistance and a fabrication method of making the same are presented. The phase change memory device includes a semiconductor substrate, a word line, an interlayer insulation film, a strapping line, a plurality of current paths, a switching element, and a phase change variable resistor. The word line is formed in a cell area of the semiconductor substrate. The interlayer insulation film formed on the word line. The strapping line is formed on the interlayer insulation film such that the strapping line overlaps on top of the word line. The current paths electrically connect together the word line with the strapping line. The switching element is electrically connected to the strapping line. The phase change variable resistor is electrically connected to the switching element.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C 119(a) to KoreanApplication No. 10-2009-0058931, filed on Jun. 30, 2009, in the KoreanIntellectual Property Office, which is incorporated herein by referencein its entirety as set forth in full.

BACKGROUND

1. Technical Field

Exemplary embodiments relate to nonvolatile memory devices. Inparticular, exemplary embodiments are concerned with phase change memorydevices, layout structures thereof, and fabrication methods thereof.

2. Related Art

One type of phase change random access memory (hereinafter, referred toas “PRAM”) is a nonvolatile memory device that exhibits differentresistivities for different corresponding solid state phases in whichthe solid state phases can be interchanged by carefully controlling theamount and/or flux of heat, usually Joule heat, imposed on thesematerials. A typical phase changeable material often comprises achalcogenide compound consisting of germanium (Ge), antimony (Sb) andtellurium (Te). Typical phase changeable materials transitionallyinterchange between into a disordered amorphous to an orderedcrystalline solid state driven by imposing Joule heat conditions. Thedisordered amorphous state usually is arbitrarily assigned to logicaldata “1” and the ordered crystalline state is assigned to logical data“0”. Alternately the disordered amorphous state can be arbitrarilyassigned to logical data “0” and the ordered crystalline state can beassigned to logical data “1”.

A memory cell of a PRAM is typically composed of a variable resistor anda switching element. The variable resistor is made of a phase changeablematerial coupled between a word line and a bit line. The switchingelement is used to selectively drive the variable resistor.

Usually in PRAM devices, word lines are provided in a form of junctionregions in a semiconductor substrate and bit lines are provided in aform of wiring. The switching elements are often times diodes ormetal-oxide-semiconductor (MOS) transistors.

It is well known that a word line of a junction region usually exhibitsa very large resistance relative to another line pattern having the samelength. Further, as the number of memory cells coupled to the word lineof junction region increase, so does the rate of word line delayincreases. This problem is especially aggravated with an increase ofintegration density of the PRAM, and as a result the word line delay ofPRAMs is prone to becoming excessively large.

Referring to FIG. 1, in order to lessen a voltage delay brought about byan increase of resistance on a word line WL of junction region, astrapping line ST_WL for applying a uniform voltage to memory cellscoupled to the word line WL is formed over bit lines BL.

This strapping line ST_WL is usually made of a relatively highconductivity metal and is electrically connected to the word line WL ofjunction region through a contact CT such as contact plug or stud. InFIG. 1, PA denotes a peripheral area and CA denotes a cell area.

However, since the strapping line ST_WL is disposed over the bit linesBL, the height of the contact CT increases by a distance between theword line WL of junction region and the strapping line ST_WL. Such anincrease of the contact height causes a signal path to exhibit anundesirable delay and makes the word line resistance increase.

Furthermore, as aforementioned, the contact CT must be provided forelectrically connect (i.e., Ohmic contact) the strapping line ST_WL withthe word line WL of junction region. In this structural configuration,since the contact CT has to occupy a space in a cell area CA and sincean interval is required between the contact CT and a bit line contact 30that electrically connects the bit line BL with a phase changeablematerial 20, then this structural configuration consequently results inenlarging a cell size of the PRAM.

SUMMARY

In an embodiment, a phase change memory device includes: a semiconductorsubstrate having cell and peripheral areas; a word line formed in thesemiconductor substrate of the cell area; a strapping line formed tooverlap with the word line; a plurality of current paths electricallyconnecting the word line with the strapping line; and a switchingelement formed on the strapping line.

In another embodiment, a phase change memory device includes: asemiconductor substrate in which cell and peripheral areas are defined;a strapping line formed on the semiconductor substrate of the cell area;a diode formed on the strapping line; a variable resistor formed on thediode and electrically connected to the diode; a bit line formed on thevariable resistor and electrically connected to the variable resistor; aword line pattern formed under the strapping line; and a current pathconfigured to electrically connect the bottom of the strapping line withthe word line pattern.

Exemplary embodiments also provide a layout structure of a phase changememory device, which may include a plurality of cell array areas inwhich pluralities of word and bit lines are arranged. The cell arrayarea may include: a strapping line overlapping with the word lines; anda plurality of current paths electrically connecting the word line tothe strapping line. The cell array areas may be continuously stackedwithout an interval. The strapping line may extend without disconnectionon the stacked cell array areas.

According to exemplary embodiments, a method of fabricating a phasechange memory device is comprised of: preparing a semiconductorsubstrate including cell and peripheral areas; forming a word-linedriving transistor in the peripheral area of the semiconductorsubstrate; forming a word line in the cell area; forming a firstinterlayer insulation film on the semiconductor substrate including theword line and the word-line driving transistor; forming a strapping lineto be electrically connected with the word line, on the first interlayerinsulation film, and a first line to be electrically connected with theword-line driving transistor; and forming a switching element on thestrapping line.

A further understanding of the nature and advantages of the presentinvention herein may be realized by reference to the remaining portionsof the specification and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present inventionwill be described with reference to the following figures, wherein likereference numerals refer to like parts throughout the various figuresunless otherwise specified. It is also understood herein that thedrawings are not necessarily to scale and in some instances proportionsmay have been exaggerated in order to more clearly depict certainfeatures of the invention. In the figures:

FIG. 1 is a section of a general PRAM device;

FIG. 2 is a section of a PRAM device according to an embodiment of thepresent invention;

FIG. 3 is a section of a PRAM device according to another embodiment ofthe present invention;

FIGS. 4 through 8 are plans of the PRAM devices according to theembodiments;

FIG. 9 is a plan of a general PRAM chip;

FIG. 10 is a plan of a PRAM chip according to an embodiment of thepresent invention;

FIG. 11 is a section of a PRAM device according to still anotherembodiment of the present invention; and

FIGS. 12 through 14 are sections illustrating a fabrication method ofthe PRAM device by an embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

Various exemplary embodiments will now be described in more detail withreference to the accompanying drawings in which some exemplaryembodiments are shown. However, specific structural and functionaldetails disclosed herein are merely representative for purposes ofdescribing exemplary embodiments. This invention, however, may beembodied in many alternate forms and should not be construed as limitedto only exemplary embodiments set forth herein. Like numbers refer tolike elements throughout the description of the figures.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of exemplary embodiments. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present. Other words used to describe therelationship between elements should be interpreted in a like fashion(e.g., “between” versus “directly between,” “adjacent” versus “directlyadjacent,” etc.).

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a,” “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes” and/or “including,” when used herein, specifythe presence of stated features, integers, steps, operations, elementsand/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components and/or groups thereof.

It should also be noted that in some alternative implementations, thefunctions/acts noted may occur out of the order noted in the figures.For example, two figures shown in succession may in fact be executedsubstantially concurrently or may sometimes be executed in the reverseorder, depending upon the functionality/acts involved.

In order to more specifically describe exemplary embodiments, variousaspects will be hereinafter described in detail with reference to theattached drawings.

An embodiment of the present invention provides a PRAM device includinga strapping line ST_WL under a switching element D.

Referring to FIG. 2, the PRAM device includes a semiconductor substrate100 in which a cell area CA and a peripheral area PA are divided by anisolation film 105. The cell area CA is prepared to accommodate memorycells (i.e., PRAM cells). The peripheral area PA is disposed outside ofthe cell area CA and prepared to accommodate driving or control elementsfor reading/writing data from/into the memory cells.

On the semiconductor substrate 100 in the peripheral area PA is formed aword-line driving transistor LXSW. The word-line driving transistor LXTWmay be composed of a gate 110, and a source 115 a and a drain 115 bwhich are placed adjacent to both sides of the gate 110. The gate 110 ofthe word-line driving transistor LXTW may be made of polycrystallinesilicon. Between the gate 110 and the semiconductor substrate 100 isinterposed a gate insulation film. On both sides of the gate 110 areformed spacers.

Over the semiconductor substrate 100 in the cell area CA, a strappingline 130 is disposed on a first interlayer insulation film 123 in whichfirst interlayer insulation film 123 is between the strapping line 130and the semiconductor substrate 100. The strapping line 130 may be madeof a high heat-resistive metal, e.g., tungsten (W). The strapping line130 is electrically connected to the source 115 a (or drain 115 b) ofthe word-line driving transistor LXSW by way of one of first plugs 125that penetrates through the first interlayer insulation film 123. Inthis embodiment, the strapping line 130 acts as a word line WL of thePRAM device.

Meanwhile, in the peripheral area PA, the gate 110 and the drain 115 brespectively contact with the first lines 135 through the first plugs125.

On the strapping line 130, a plurality of diodes D, that act asswitching elements, is arranged in a substantially uniform interval. Thediodes D are electrically isolated away from each other through a secondinterlayer insulation film 140. Here, if the strapping line 130 is madeof polycrystalline silicon, the diode D becomes a silicon Schottkydiode. If the strapping line 130 is made of a metal, the diode D becomesa metallic Schottky diode.

Heat electrodes 150 are laid respectively on the diodes D. The heatelectrode 150 may have a diameter smaller than the diode D and may beformed of a relatively high resistivity conductor. While this embodimentadopts the heat electrode 150 in a cylindrical shape, it is understoodherein that this configuration is not restrictive hereto and othergeometric configuration patterns are envisioned and thus available forthe heat electrode 150.

Further, although not shown in the drawing, an ohmic contact layer maybe formed on the diode D when the heat electrode 150 is made of ametallic material.

Variable resistors Rv are placed respectively on a third interlayerinsulation film 145 that includes the heat electrodes 150. The variableresistors Rv may be formed of a stacked film composed of a phasechangeable film 160 and a top electrode 165. The phase changeable film160 is where a solid phase change can be reversibly transitioned into aset or reset state in response to a current supplied between the heatelectrode 150 and the top electrode 165. The heat electrode 150 and thetop electrode 165 may be made of the same or different type ofconductive material from each other.

A fourth interlayer insulation film 170 is formed by covering thestructure that includes the variable resistors Rv. Bit lines 180 arearranged on the fourth interlayer insulation film 170 to overlap thecorresponding variable resistors Rv. The variable resistors Rv and thebit lines 180 are electrically connected to each other by way of secondplugs 175.

In the peripheral area PA, second lines 182 are arranged on the thirdinterlayer insulation film 170 and the second lines 182 are electricallyconnected to the first lines 135 by way of the third plugs 178.

According to this embodiment, the word line is formed in a conductivepattern, which has lower resistance than in the junction region, underthe switching element. Thus, there is no need of preparing an additionalregion for word line contact in the presently embodied PRAM device.

In the structure shown in FIG. 2, as the first interlayer insulationfilm 123 is only provided under the strapping line 130, the lowerstructure would be unstable and prone to causing a dishing effectbrought about by a chemical-mechanical polishing (CMP) process.

For this reason, referring to FIG. 3, a PRAM device may be embodiedfurther including a word line pattern 120 supporting the strapping line130, and a plurality of current paths 127.

In detail, the word line pattern 120 is formed in the semiconductorsubstrate 100 under the strapping line 130. The word line pattern 120may be configured in the substantially the same structure with the gate110 of the word-line driving transistor LXSW, which can be used tosubstantially act as a word line in this embodiment. The strapping line130 is formed to overlap the word line pattern 120.

The current paths 127 function as electrically connections between thestrapping line 130 and that of the word line pattern 120. The currentpaths 127 may be formed of the same material as with the first plugs125. Accordingly, the word line pattern 120 is electrically connected tothe strapping line 130 by way of the current paths 127.

One variation of the word line pattern 120, as illustrated in FIG. 4, isthat it can be configured in a linear shape and may be disposed in aparallel pattern and may be aligned to overlap the strapping line 130.

On the other hand, referring to FIG. 5 through 7, word line patterns 120a may be aligned in an island type arrangement pattern. The word linepattern 120 a of the island type arrangement pattern may be formed inplurality to a single one of the strapping line 130 with a uniforminterval.

For example, as shown in FIGS. 5 and 6, one of the word line pattern 120a can be aligned to only overlap with an even member of the strappinglines 130 by being placed between others word line patterns 120 a thatare aligned to only overlap with an odd member of the strapping lines130.

Further, as shown in FIG. 7, the word line patterns 120 a of islands maybe shiftingly disposed in a predetermined interval along the order ofthe strapping lines 130. Thus, the word line patterns 120 a are seen asarranged in a step on plane.

Otherwise, referring to FIG. 8, there may be provided word line patterns120 b that are arranged in a symmetrical matrix pattern of islands, eachword line pattern 120 b overlapping two adjacent members of thestrapping lines 130. And, the current path 127 contacts with only one ofthe strapping lines 130 that are overlapped thereon. In this embodimentone grouping of word line patterns 120 b is arranged along the strappingline 127 so that this grouping is electrically connected to the upperoverlapped strapping lines 130 by way of the current path 127, whileanother grouping of the word line patterns 120 b is electricallyconnected to the lower overlapped strapping lines 130 by way of thecurrent path 127.

The current path 127, as shown in FIGS. 4, 5 and 8, may be formed in adiameter (line width) smaller than a width (w) of the strapping line130. Further, as illustrated in FIG. 6, the current path 127 may beconfigured to have its upper side shaped in a slit pattern. Otherwise,the current path 127, as shown in FIG. 7, may be formed in a diameterlarger than the width of the strapping line 130.

Accordingly, the combination of the current paths 127 and the word linepattern 120 acts as a shunting layer that more uniformly branches out acurrent flow which reduces equivalent resistance differences between theword line (the structure including the word line pattern, the currentpath and the strapping line).

In addition, as can be seen from FIG. 3, since the word line pattern 120is leveled at a predetermined height, the distance between the word linepattern 120 and the strapping line 130, i.e., a height (or thickness) ofthe current path 127 becomes substantially smaller with regards to thatof the height of the contact CT as depicted in FIG. 1. Thus with thisstructure of the present PRAM device a reduction in the word lineresistance can be realized.

Furthermore, since the current paths 127 electrically connecting thestrapping line 130 and the word line pattern 120 are all placed underthe strapping line 130, then there is no need for an additional contactregion and for a spatial margin thereof for linking the strapping line130 with a dummy gate 120.

With this structure of the PRAM device according to these exemplaryembodiments of the present invention, the word line pattern 120 and theplural current paths 127 contribute to realizing a longer word lineexhibiting a given specification of resistance.

In further detail, as shown in FIG. 9, a general PRAM chip 200 is can beconfigured in a cell array area CAA and in a peripheral circuit fieldPCA. The cell array area CAA may include the cell area CA and theperipheral circuit field PCA may include the peripheral area PA.

A row control block X-CON may be disposed in the peripheral circuitfield PCA and be coupled to word lines that extend along an X-directionin the cell array area CAA. A column control block Y-CON may be disposedin the peripheral circuit field PCA and be coupled to bit lines thatextend along a Y-direction in the cell array area CAA. The row controlblock X-CON may include word-line driving transistors LXSW and thecolumn control block Y-CON may include bit-line driving transistorsLYSW. The row and column control blocks, X-CON and Y-CON, are requiredto be apart from the cell array area CAA in a predetermined interval d.

Different from the disposition shown in FIG. 9, in this embodiment,since the word line pattern 130 and the current paths 127 are formedunder the strapping line 130 of the cell array area CAA, then theresistive characteristic of the word line can be highly improved topermit a length of the word line to be longer. Thereby, it is possibleto increase the number of memory cells (i.e., the number of diodes andvariable resistors) per word line. In exemplary embodiments of thepresent invention, a length of word line may mean a length of thestrapping line 130.

Regarding such a condition, referring to FIG. 10, a plurality of cellarray areas CAA is continuously arranged and the word lines, i.e., thestrapping lines 130, are disposed to intersect the cell array areas CAAwithout any division. This structure that the strapping lines 130 aredisposed to intersect a series of the memory array areas withoutdisconnection will be referred to as “stacked cell array”.

The row control blocks X-CON and the column control blocks Y-CON,respective to the plural cell array areas CAA, are each consolidated anddisposed at the periphery of the stacked cell array area. The unifiedrow and column control blocks disposed at the periphery of the stackedcell array area may have an area corresponding to summing the dividedrow and column control blocks X-CON and Y-CON.

By stacking the plural cell array area CAA and consolidating the row andcolumn control blocks at the fringes of the stacked cell array area, itis able to shorten the distances d between the cell array areas CAA andthe row control blocks X-CON, reducing an area of the PRAM chip.

Further, referring to FIG. 11, it is possible to accomplish a shuntingeffect by forming the word line WL in a form of junction region (i.e., ajunction word line), without the word line pattern 120, and simplyforming the plural current paths 127 between the strapping line 130 andthe junction word line.

Now hereinafter will be described a fabrication method of the PRAMdevice by the exemplary embodiments in conjunction with FIGS. 12 through14.

First, referring to FIG. 12, the field isolation film 105 is selectivelyformed in the semiconductor substrate 100 by means of any number of wellknown processes such as field oxidation. Thereby, the cell area CA andthe peripheral area PA are confined and defined in the semiconductorsubstrate 100.

Next, after sequentially stacking a gate insulation film 106 and aconductive gate layer 107 on the semiconductor substrate 100, theconductive gate layer 107 and the gate insulation film 106 areselectively patterned and partly removed in the cell and peripheralareas CA and PA. Then, an insulation film is uniformly depositing on theresultant structure of the semiconductor substrate 100 andanisotropically etched so that both sides of the patterned conductivegate layer 107 and gate insulation film 106 are covered to form thespacer insulation films 108. Thereby, the gate 110 of the word-linedriving transistor LXSW is structured in the peripheral area PA, whilethe word line pattern 120 is formed in the cell area CA. Impurities areimplanted into the semiconductor substrate 100, at both sides of thegate 110, to form the source 115 a and the source 115 b. Thereby, theword-line driving transistor LXSW is completed in the peripheral areaPA. Forming the source 115 a and the drain 115 b may be carried outunder the condition of exclusively opening the peripheral area PA.

During this, as illustrated in FIG. 11, in the structure without theword line pattern 120, the junction word line WL is formed by patterningthe gate insulation film 106 and patterning the conductive gate layer107 so as to partly leave them only in the peripheral area PA. The stepof forming the source 115 a and the drain 115 b can be performed at thesame time by selectively implanting impurities into the cell area CA.

Continuously, referring to FIG. 13, the first interlayer insulation film123 is deposited on the resultant structure of the semiconductorsubstrate 100 which already includes the word-line driving transistorLXSW. The first interlayer insulation film 123 may be made of siliconoxide. Then, the first interlayer insulation film 123 is selectivelyetched away to form (not shown) contact holes which partly expose theupper side of the word line pattern 120, the gate 110, the source 115 aand the drain 115 b of the word-line driving transistor LXSW. Thecontact holes are subsequently filled in with a conductive layer to formthe first plugs 125 and to form the current paths 127. A conductivelayer forming the first plugs 125 and the current paths 127 may be madeof doped polycrystalline silicon or metal such as tungsten (W).

Next, after depositing a first conductive layer on the first interlayerinsulation film 123 including the first plugs 125 and the current paths127, the first conductive layer is patterned to form the strapping line130 and the first lines 135. The strapping line 130 is electricallyconnected to the first plugs 125 that are electrically connected to thecurrent paths 127 and the source 115 a of the word-line drivingtransistor LXSW. The first lines 135 are electrically connected to thegate 110 and to the drain 115 b, respectively, of the word-line drivingtransistor LXSW. The first conductive layer forming the strapping line130 and the first lines 135 may be made of a high heat-resistive andhigh-conductive metal, e.g., tungsten (W).

Then, referring to FIG. 14, after depositing the second interlayerinsulation film 140 on the resultant structure of the semiconductorsubstrate 100 that includes the strapping lines 130 and the first lines135, the second interlayer insulation film 140 is selectively etchedaway to partly expose the strapping line 130 and to form a plurality ofdiode contact holes (not shown). The diode contact holes aresubsequently filled in with a polycrystalline silicon film doped withN-type impurities. The diodes D are formed by implanting P-typeimpurities into the doped polycrystalline silicon film of the diodecontact holes. The diode D are laid on the strapping line 130 and madeof a polycrystalline material formed by deposition, and subsequentlybecome Schottky diodes. While this embodiment is described as formingthe diodes D with a polycrystalline silicon film from the diode contactholes, it is also permissible to, after completing the diodes D bypatterning the polycrystalline film, deposit the second interlayerinsulation film 140 over the structure of the diodes D.

Next, the third interlayer insulation film 145 is formed on the secondinterlayer insulation film 140 including the diodes D. The thirdinterlayer insulation film 145 may be made of high heat-resistivesilicon nitride. Then, the third interlayer insulation film 145 isselectively etched away to expose the tops of the diodes D to formheat-electrode contact holes (not shown). These heat-electrode contactholes are subsequently filled in with a conductive film to form the heatelectrodes 150. The heat electrode 150 may be made of any number ofvarious kinds of high heat-generative conductors such as dopedpolycrystalline silicon, silicon germanium (SiGe), titanium nitride,etc., e.g., a high resistivity conductor. If the heat electrode 150 ismade up of a metal film, it is herein envisioned to further form ohmiccontact layers (not shown) on the tops of the diodes D before depositingthe third interlayer insulation film 145.

Afterward, the phase changeable film 160 and the top electrode layer 165are sequentially stacked on the third interlayer insulation film 145.The phase changeable film 160 may be made of a chalcogenide containingcompound. The top electrode layer 165 and the phase changeable film 160are then subsequently patterned to contact with the heat electrodes 150and to form the variable resistors Rv.

In subsequent processing steps, referring to FIGS. 2 and 3, the fourthinterlayer insulation film 170 is deposited on the structure of thesemiconductor substrate 100 that have the variable resistors Rv. Afterforming via holes (not shown) to expose the variable resistors Rv of thecell area CA and the first lines 135 of the peripheral area PA, the viaholes are then filled in with a conductive layer to form the secondplugs 175 act as bit line contacts and as the third plugs 178. Next, byusing any number of well known processing techniques, the bit lines 180are formed to contact with the second plugs 175 and the second lines 182are formed to contact with the third plugs 178.

As described above in detail, for the purpose of reducing the resistanceof the word line, the strapping line composed of relatively highlyconductive material is formed under the switching element, i.e., thediode, and the current paths are provided between the word line and thestrapping line. Therefore, a reduction in the resistance of the wordline can be realized by way of the highly conductive strapping line andthe resistance can be further reduced by way of the plural current pathsthat contribute to helping signal transmission through the word line.

In addition, according to the exemplary embodiments stated above, sincethe word line is configured in a form of wiring by the same manner withthe gate of the word-line driving transistor, it is also effective inimproving the resistance characteristics of the word line, as well asshortening a distance to the strapping line and reducing the resistanceof the word line in itself.

Such improvement to the present PRAM resistance characteristics maypermit the corresponding word line to be extended because it exhibits alower resistance and as a result enables more cell array areas to bestacked along the word line. Thus, it is possible to realize a betterarea margin for chip size in the extension of the word line.

Further, since the current paths and the word line of wiring type can beformed at the same time with the gate of the word-line drivingtransistor and the plugs and the strapping line can be formed at thesame time with the first line, then there is no need to carryout out anadditional processing step for them.

Moreover, since the strapping line formed in the cell area of the PRAMdevice is sustained by the current paths, i.e., a plurality of plugs, adishing effect is less likely to occurs subsequent to a CMP process.

The foregoing is illustrative of exemplary embodiments and is not to beconstrued as limiting thereof. Although a few exemplary embodiments havebeen described, those skilled in the art will readily appreciate thatmany modifications are possible in exemplary embodiments withoutmaterially departing from the novel teachings and advantages.Accordingly, all such modifications are intended to be included withinthe scope of this invention as defined in the claims.

1. A phase change memory device, comprising: a semiconductor substrateincluding cell and peripheral areas separated by an isolation film; aword line formed in the cell area of the semiconductor substrate; aninterlayer insulation film formed on the word line; a strapping lineformed on the interlayer insulation film such that the strapping lineoverlaps on top of the word line; a plurality of current pathselectrically connecting the word line with the strapping line; aswitching element formed on top of the strapping line; and a phasechange variable resistor formed over the switching element, wherein thephase change variable resistor is electrically connected to theswitching element, wherein the word line is configured in an island typearrangement pattern that is partly overlapped by the strapping line. 2.The phase change memory device according to claim 1, wherein the phasechange variable resistor comprises a chalcogenide containing compound.3. The phase change memory device according to claim 1 further comprisesa word-line driving transistor formed in the peripheral area of thesemiconductor substrate.
 4. The phase change memory device according toclaim 3, wherein the word-line driving transistor comprises a gatebetween a source and a drain.
 5. The phase change memory deviceaccording to claim 4, wherein the strapping line is electricallyconnected to the source of the word-line driving transistor.
 6. Thephase change memory device according to claim 4, which furthercomprises: first plugs formed on and electrically connected to the gateand the drain; and first lines formed on and electrically connected tothe first plugs such that the gate and the drain of the word-linedriving transistor are electrically connected to respectivecorresponding first lines.
 7. The phase change memory device accordingto claim 1, wherein the switching element is a diode.
 8. The phasechange memory device according to claim 1, further comprising a bit lineformed over and electrically connected to the phase change variableresistor.
 9. The phase change memory device according to claim 6 whereinthe strapping line, the current paths, the first plugs and the firstlines are formed from a single conductive layer.
 10. The phase changememory device according to claim 1 further comprising a heat electrodeelectrically connected to the switching element and to the phase changevariable resistor.
 11. The phase change memory device according to claim1 further comprising a bit line electrically connected to the phasechange variable resistor.